In studies of SV40 early antigens, Ad2(plus) ND2 induced in infected cells two SV40 specific polypeptides of 42,000 and 56,000 molecular weight; Ad2(plus) ND1 induced a 28,000 molecular weight peptide. The Ad2(plus) ND1 segment is found to be sufficient to induce the SV40-specific tumor antigen in mice. With 2-dimensional gel electrophoresis a 4th AAV structural protein (VP) was detected. Each of the four VP's (m.w. 86.5, 72.5 61.5, 60.5) contains multiple isoelectric species. While VP 1, 2 and 3 share many tryptic peptides, at least, VP 1 and 3 each contain unique tryptic peptides. Virion proteins as well as cell free translated proteins are presently being analyzed to test whether the VP's are overlapping or whether that frame shift also plays a part apparently as a method of conserving DNA by this virus.